ULTRA-FINE CEMENTED CARBIDE Ni BINDER PHASE AND TOOL USING THE SAME

ABSTRACT

Provided is a high hardness ultra-fine cemented carbide with a Ni binder phase for a wear resistant tool. An ultra-fine cemented carbide having high specularity and/or high strength, high hardness, and high wear resistance is obtained by using an ultra-fine raw powder of WC, controlling the amount of Ni, and the contents of V and Cr, so that a third phase containing V and Cr precipitates in a microstructure of the cemented carbide in a finely dispersed state, and at the same time, the size of Ni pool is controlled to a value equal to or less than the average grain size of WC. By using this cemented carbide, the range of application to an aspherical glass lens mold, an ultrahigh pressure generation container for neutron diffraction experiment, a non-ferromagnetic corrosion resistant and wear resistant tool, and the like is expanded.

BACKGROUND OF THE INVENTION

The present invention relates to a WC based ultra-fine cemented carbide with a Ni binder phase which achieves excellent specularity easily, has a higher level of radiation safety, and has a long lifetime, especially when used as an aspherical glass lens mold, an ultrahigh pressure generation container for neutron diffraction experiment, a non-ferromagnetic wear resistant tool requiring high corrosion resistance and high wear resistance, or the like.

Among wear resistant tools, for uses requiring higher specularity, a WC based cemented carbide not comprising a binder phase is used. Moreover, for ultrahigh pressure generation containers used for studying characteristics of a material under high pressure (an anvil, a cylinder, or the like), a WC based cemented carbide and a WC based ultra-fine cemented carbide comprising a Co binder phase in a relatively small amount are used. Furthermore, a WC-VC-Cr₃C₂-10 mass % Ni ultra-fine cemented carbide comprising 10 mass % of Ni as a binder phase is used for ultrahigh pressure generation containers requiring high strength, and non-ferromagnetic, highly corrosion resistant, and highly wear resistant tools.

SUMMARY OF THE INVENTION

Among cemented carbides used for wear resistant tools, molds used for forming aspherical glass lenses need high oxidation resistance, high hardness, and high specularity because the aspherical glass lens is formed by pressing a blank ball of the glass lens at a high temperature of about 600° C. Especially, as for the high specularity, surface roughness of a few nm Ra is required. For achieving this, a cemented carbide used for such molds is designed not to comprise a binder phase or to comprise a binder phase in an extremely small amount.

A major one is a cemented carbide comprising WC and a solid solution composite carbide phase of TIC-WC disclosed in Patent Literature 1 (Japanese Patent Appl. Laid-Open No. H2-120244). The cemented carbide comprising WC and a solid solution composite carbide phase of TiC-WC has excellent oxidation resistance and high hardness, and most importantly, has excellent specularity because the cemented carbide does not comprise a binder phase. Furthermore, the cemented carbide has satisfactory sinterability because of the solid solution composite carbide phase of TiC-WC, and can be produced by ordinary sintering and Hot Isostatic Press (hereinafter, referred to as HIP), thereby has satisfactory productivity. However, it is sometimes difficult to process the cemented carbide because of inferior grindability of the solid solution composite carbide phase of TiC-WC, that is, higher hardness and slightly higher brittleness in comparison with WC.

Then, a cemented carbide not comprising the solid solution composite carbide phase of TiC-WC is demanded, and Patent Literature 2 (Japanese Patent No. 3310138) discloses a WC-W₂C_(x) based cemented carbide which uses WC having a grain size of 0.5 μm or less. However, the disclosed cemented carbide requires hot press and can not be produced by ordinary vacuum sintering, which results in lower productivity.

Patent Literature 3 (Japanese Patent Appl. Laid-Open No. 2004-91241) discloses a WC based cemented carbide composed of a single phase of WC having a grain size of less than 0.3 μm; however, the disclosed cemented carbide is produced by a pulsed-current pressure sintering process, which results in remarkably lower productivity.

Patent Literature 4 (Japanese Patent Appl. Laid-Open No. H5-59405) discloses a traditional WC based ultra-fine cemented carbide which can be produced by pre-sintering and HIP, thereby has satisfactory productivity. This is a WC-2.0 to 7.0 mass % (Mo or Mo₂C)-0.2 to 0.6 mass % VC-0.2 to 1.0 mass % Co cemented carbide which uses WC having a grain size of 2 μm or less. However, the cemented carbide is a Co based cemented carbide, and therefore, the cemented carbide is applied to a material for a high pressure water flow nozzle, or for a tool such as cutting die, sliding die, or drawing die, but application of the cemented carbide to an aspherical glass lens mold is not disclosed. It is believed that this is attributed to the fact that the disclosed cemented carbide has lower oxidation resistance in comparison with the aforementioned cemented carbide comprising WC and a solid solution composite carbide phase of TiC-WC, the aforementioned WC-W₂C_(x) based cemented carbide, and the aforementioned cemented carbide composed of a single phase of WC, due to its composition.

Here, it is expected that the oxidation resistance is improved by replacing Co by Ni. However, such replacement by Ni is difficult to be turned into actual utilization because, as described in Non Patent Literature 1 (Suzuki Hisashi, Terada Osamu, Ike Hiroyuki: Journal of The Japan Society of Powder and Powder Metallurgy, 42 (1995), p. 1341, (in Japanese)), unlike the case of Co, Ni is liable to form aggregated particles having a large size in the course of pulverizing•mixing a raw powder, and even when Ni is added in an extremely small amount of about 0.12 to 0.3 mass %, the added Ni forms a coarse pore in the obtained cemented carbide, which provides a coarse Ni phase (referred to as Ni pool) having a larger size than the grain size of WC after HIP, thereby the high specularity cannot be obtained.

That is, for an aspherical glass lens mold, there is provided no material having all of satisfactory productivity, satisfactory processability, and high oxidation resistance.

Next, as for an ultrahigh pressure generation container used for creating a high pressure of 5 GPa or higher, this tool requires high strength and high hardness, and therefore, a WC-Co based cemented carbide comprising a binder phase in a small amount of 5 to 6 mass % is generally used. Furthermore, in the case where basic properties of a substance are investigated under an ultrahigh pressure of more than 6 GPa, a WC-VC-Cr₃C₂-5 to 10 mass % Co ultra-fine cemented carbide with higher hardness is used.

Here, neutron diffraction is performed in order to investigate the basic properties of a substance, and during the neutron diffraction, neutron beams penetrate through the ultrahigh pressure generation container made of the ultra-fine cemented carbide. On this occasion, if the binder phase is composed of Co (commercially available Co is ⁵⁹Co at 100 at %), ⁵⁹Co turns into ⁶⁰Co which is a radioactive isotope to emit γ-rays. Accordingly, handling of the ultrahigh pressure generation container made of the ultra-fine WC-Co cemented carbide after experiment becomes extremely dangerous for operators.

Then, when a binder phase is composed of Ni, a radioactive isotope ⁶³Ni is generated. However among Ni, ⁶²Ni which turns into ⁶³Ni is contained at only about 4 at % in a commercially available Ni, and ⁶³Ni emits only β-rays, i.e. negative electrons which are not so much dangerous compared with γ-rays. Since the remaining 96 at % of Ni does not emit radiation or has an extremely short half-life period, Ni has remarkably smaller radiation danger than Co. Therefore, it is demanded that the material of the ultrahigh pressure generation container for neutron diffraction experiment is an ultra-fine cemented carbide comprising a binder phase not of Co but of Ni. The data on isotopes of Co and Ni are sourced from Non Patent Literature 2 (Chemistry Handbook—Basic Edition—, 5th revised version, Maruzen Publishing Co., Ltd, 2004, p. I-43 to I-44, (in Japanese)).

That is, for the ultrahigh pressure generation container used for neutron diffraction experiment, a WC-VC-Cr₃C₂—Ni based ultra-fine cemented carbide comprising a small amount of a binder phase is demanded rather than a WC-VC-Cr₃C₂—Co based ultra-fine cemented carbide. However, as mentioned above, unlike the case of Co, Ni tends to form aggregated particles having a large size during pulverizing and mixing a raw powder. And the aggregated particle causes the generation of a coarse pore in the obtained cemented carbide and then leads to a Ni pool having a larger size than the grain size of WC after HIP. As a result, the high specularity is impaired in the case where Ni is added in an extremely small amount as mentioned above, or in the case where Ni is added in an amount of a few percent, a large Ni pool is generated, and then, high strength is hard to be obtained. The WC-VC-Cr₃C₂—Ni based ultra-fine cemented carbide comprising a small amount of a binder phase fails to achieve high strength, and only the WC-VC-Cr₃C₂-10 mass % Ni cemented carbide disclosed in Non Patent Literature 3 (Suzuki Hisashi, Terada Osamu, Ike Hiroyuki: Journal of The Japan Society of Powder and Powder Metallurgy, 42 (1995), p. 1345, (in Japanese)) is used for the ultrahigh pressure generation container used for neutron diffraction experiment.

Here, Patent Literature 5 (Japanese Patent Appl. Laid-Open No. S64-52043) discloses a WC-0.2 mass % Cr-0.6 mass % Ta-5 mass % Ni; however, the disclosed cemented carbide has a low transverse-rupture strength of 220 kgf/mm² (2.16 GPa). The reason for this is believed to be that the dimension of the Ni pool can not be controlled because the disclosed cemented carbide is a low-Ni content cemented carbide. Moreover, Japanese Patent Appl. Laid-Open No. S64-52043 discloses that grain growth inhibitors (Cr₃C₂, VC, TaC, NbC, and the like) are contained in such an amount that they do not precipitate as a third phase.

The technique disclosed in Non Patent Literature 3 (Suzuki Hisashi, Terada Osamu, Ike Hiroyuki: Journal of The Japan Society of Powder and Powder Metallurgy, 42 (1995), p. 1345, (in Japanese)) uses removing the aggregated powder by a #1000 sieve and performing high temperature and high pressure HIP at 1500° C. and 1500 atm (147 MPa) as a means for removing the Ni pool, thereby achieves a transverse-rupture strength of 4.4 GPa for a WC-0.4 to 0.6 mass % VC-10 mass % Ni; however, the hardness remains at 1800 HV. Moreover, there are found no results obtained by using a lower Ni content. Furthermore, it is disclosed that sieving by the #1000 sieve only enables removal of an aggregated powder having a size of 20 to 30 μm, and that using a composition and/or sintering temperature which provide less viscous fluidity of Ni leads to a lower strength. That is, the reason why there are found no results obtained by using a lower Ni content is that, when a low Ni content is used, viscous flowing of Ni becomes less easy at an ordinary sintering temperature, which provides a coarse pore (becoming a Ni pool after HIP) and then high strength is difficult to be obtained.

Also, Patent Literature 6 (Japanese Patent Appl. Laid-Open No. S56-130450) discloses 3 to 30 mass % Ni in claims; however, Examples only refer to WC-0.2 to 0.6 mass % VC-10 mass % Ni. Moreover, it is disclosed that the maximum content of V dissolved in the binder phase is 6.5 mass %; however, combined addition of V and Cr is not disclosed.

Other than the ultrahigh pressure generation container for neutron diffraction experiment, the WC-VC-Cr₃C₂-10 mass % Ni cemented carbide is used for ultrahigh pressure generation containers requiring high strength; however, a cemented carbide comprising a smaller amount of a binder phase is not used due to insufficient strength. Also, for non-ferromagnetic, corrosion resistant, and wear resistant tools, that is, the ultrahigh pressure generation container, mold, die, punch, cutter, dies, plug, plate, and the like, the WC-VC-Cr₃C₂-10 mass % Ni cemented carbide is used; however, a cemented carbide comprising a smaller amount of binder phase is hardly used because such cemented carbide often has no sufficient strength.

It is noted that, in any of the conventional techniques, contents of VC and Cr₃C₂ are limited to such small amounts that they are below solid solubility into Ni, and it is not supposed that the contents are such large amounts that a third phase containing V and Cr precipitates.

Also, the present inventors produced a WC-VC-Cr₃C₂-3 to 9 mass % Ni based ultra-fine cemented carbide by ordinary mixing-and-pulverizing, drying, pressing, sintering, and HIP, and then, a Ni pool of a few micrometers was generated. FIG. 1 shows optical microscopic observation of the specular surface of a cemented carbide which has a composition of WC-0.2 mass % VC-0.3 mass % Cr₃C₂-3 mass % Ni and medium carbon, and is obtained by performing ordinary mixing-and-pulverizing, sintering at 1440° C. for 1 hour, and HIP at 1440° C. and 100 MPa. The white phase is a Ni pool. That is, it was confirmed that a WC-VC-Cr₃C₂—Ni based ultra-fine cemented carbide without a Ni pool could not be produced easily when the cemented carbide comprised less than 10 mass % of Ni as a binder phase.

Next, the technique disclosed in Patent Literature 7 (Japanese Patent Appl. Laid-Open No. 2008-38242) invented by the present inventors achieves higher strength and higher hardness than conventional techniques by finding appropriate contents of VC and Cr₃C₂ and then allowing the WC-VC-Cr₃C₂—Co cemented carbide to have an average grain size of WC of 0.3 μm or less. The average grain size of WC of 0.3 μm or less leads to high specularity in the case of the aspherical glass lens mold, advantageously leads to high strength and high hardness in the case of the ultrahigh pressure generation container for neutron diffraction experiment, and allows the non-ferromagnetic, corrosion resistant, and wear resistant tool to have higher strength and higher hardness, and then, to have a longer lifetime; however, nothing is disclosed about a WC-VC-Cr₃C₂—Ni based cemented carbide.

That is, if a WC-VC-Cr₃C₂—Ni based ultra-fine cemented carbide comprises Ni in a smaller amount than the WC-VC-Cr₃C₂-10 mass % Ni cemented carbide and has an average grain size of WC of 0.3 μm or less so as not to contain a Ni pool, such WC-VC-Cr₃C₂—Ni based ultra-fine cemented carbide can be expected to have superior performance than those of the conventional techniques as the aspherical glass lens mold, the ultrahigh pressure generation container for neutron diffraction experiment, and the non-ferromagnetic, corrosion resistant, and wear resistant tool.

Therefore, at first, the present inventors have diligently studied to produce a WC-VC-Cr₃C₂—Ni based ultra-fine cemented carbide without allowing Ni to aggregate by mixing-pulverizing. Next, the present inventors have diligently studied to control the average grain size of WC to be 0.3 μm or less.

As a means not to allow Ni to aggregate, at first, Ni compound powders, for example NiO powder, instead of Ni powder, was used. NiO powder is an oxide powder and therefore could be easily pulverized, and the present object could be achieved in the case where NiO powder is added in an extremely small amount. However, an oxide is liable to generate a pore resulting from oxygen in the cemented carbide, and NiO is unpreferable from a view point of industrial safety and health because NiO is a carcinogen substance. However, by this attempt, it was found that a coarse aggregated Ni powder was less generated when Ni became brittler. This is the first finding.

Then, a WC-VC-Cr₃C₂-3 to 6 mass % Ni ultra-fine cemented carbide was produced by strongly more pulverizing the carbide and Ni powders preliminarily and inducing work-hardening, and then, using it as the raw powder containing Ni. As a result, as expected, it was found that the coarse aggregated powder was not generated and an excellent low-binder WC-VC-Cr₃C₂-3 to 6 mass % Ni cemented carbide in which the Ni pool had a maximum size of 0.3 μm or less could be obtained. This is the second finding. Here, the size of the Ni pool is a value obtained by measuring a long axis in an optical microscope photograph or a SEM photograph. Moreover, that the maximum size of the Ni pool is smaller than the average grain size of WC is confirmed by the fact that a Ni pool having a larger size than the average grain WC does not act as a fracture source in a transverse-rupture strength test piece.

Next, the present inventors have diligently studied to achieve higher strength and higher hardness than the conventional techniques by allowing the WC-VC-Cr₃C₂—Ni cemented carbide to have an average grain size of WC of 0.3 μm or less.

At first, as a result of diligent study on the effect of the average grain size of WC in the WC-VC-Cr₃C₂—Ni ultra-fine cemented carbide on a size of the Ni domain (domain is a crystal grain of a binder phase, that is, a region with the same crystal orientation), it was found that a size of the Ni domain became finer with decreasing of the average grain size of WC as shown in FIG. 3. Here, the size of the Ni domain is a value obtained by measuring a long axis in an optical microscope photograph. This is the third finding. In FIG. 3, an open circle represents a case where sintering is performed under ordinary sintering conditions at 1450° C. for 1 hour and cooling is performed at 0.07° C./s after sintering, and a filled circle represents a case where sintering is performed under ordinary sintering conditions at 1450° C. for 1 hour and cooling is performed at 0.23° C./s after sintering.

Furthermore, as a result of diligent study on the effect of contents of V and Cr on a microstructure of the WC-VC-Cr₃C₂—Ni based ultra-fine cemented carbide, it was found that, in a cemented carbide comprising 3 to 9 mass % Ni, the third phase containing V and Cr was dispersed in the microstructure of the cemented carbide with a maximum size of 0.3 m or less even when V and Cr were added in such large amounts as 5 to 15 mass % with respect to Ni in the case of V and 11 to 50 mass % with respect to Ni in the case of Cr, these values being larger than those in the conventional techniques.

The conventional techniques have avoided comprising the third phase containing V and Cr in the microstructure. This is because a coarse third phase containing V and Cr will be a fracture origin, which reduces the transverse-rupture strength, and then, it becomes difficult to obtain high strength.

However, as was found first, since the Ni domain becomes finer with decreasing of the average grain size of WC, the amounts of V and Cr precipitating from a liquid phase region which forms one domain are reduced, thereby it is possible to finely disperse the third phase containing V and Cr precipitating at a quadruple point or a quintuple point of the domain.

This has achieved precipitation of the third phase containing V and Cr in a finely dispersed state in the WC-VC-Cr₃C₂—Ni based ultra-fine cemented carbide, and it has been found that a decrease in the strength due to the precipitation of third phase containing V and Cr can be avoided. This is the fourth finding. Needless to say, adding VC and Cr₃C₂ in such amounts that the third phase containing V and Cr precipitates is effective in inhibiting abnormal growth of WC. Moreover, fine dispersion of the third phase containing V and Cr is also effective for the hardness.

The study was further continued for the contents of V and Cr in the case of a cemented carbide comprising an extremely small amount of a binder phase and comprising Ni in an amount of 0.12 to 0.3 mass % and as a result, it was found that the third phase containing V and Cr could be finely dispersed with a maximum size of 0.3 μm or less even when V and Cr were added to the binder phase in such large amounts as 30 to 100 mass % in the case of V and 130 to 300 mass % in the case of Cr with respect to Ni contained in an amount of 0.12 to 0.3 mass %.

This is presumably attributed to the fact that aggregation of Ni does not occur because of pre-pulverization, which improves the sinterability, thereby the Ni pool resulting from the pore is not generated by ordinary sintering. This is also presumably attributed to the fact that an extremely smaller amount of Ni leads to a decreased amount of the liquid phase at a sintering temperature, and then, the third phase can exist almost in a solid phase state and/or that the Ni domain becomes extremely small, and then, the amount of the third phase precipitating from a liquid phase region which forms one domain becomes smaller, thereby the third phase easily precipitates in a remarkably finely dispersed state in a cooling step after sintering. This is the fifth finding.

Here, that the maximum size of the third phase containing V and Cr is smaller than the average grain size of WC is confirmed by the fact that a third phase containing V and Cr having a larger size than the average grain size of WC does not act as a fracture origin in a transverse-rupture strength test piece. The size of the fracture origin in a transverse-rupture test piece is a value obtained by measuring a long axis in a SEM photograph.

The fourth and fifth findings are different from the conventional techniques in which the contents of VC and Cr₃C₂ are controlled to be lower values so that the third phase containing V and Cr does not precipitate.

Upon the aforementioned findings, the present inventors has invented a WC-VC-Cr₃C₂—Ni based ultra-fine cemented carbide comprising WC as a main component and 0.12 to 9 mass % of Ni, wherein V is contained in an amount of 5 to 100 mass % with respect to the mass of Ni, and Cr contained in an amount of is 11 to 300 mass % with respect to the mass of Ni, so that a third phase containing V and Cr is dispersed in a structure of the cemented carbide with a maximum size of 0.3 μm or less, and the cemented carbide has an average grain size of WC of 0.3 μm or less, and has a size of a Ni pool equal to or less than the average grain size of WC, and a hardness of 2100 to 2800 HV10.

Based on the aforementioned findings, the glass lens mold, the ultrahigh pressure generation container for neutron diffraction experiment, and the general non-ferromagnetic, corrosion resistant, and wear resistant tool have been further developed. As a result, it was found that the each tool had an appropriate amount of the binder phase as described below, and a composition appropriate for each tool was invented by defining the amount of the binder phase, while it was appropriate that the each tool used the same process; the Ni based ultra-fine cemented carbide was produced by strongly more pulverizing a carbide and Ni preliminarily and inducing work-hardening, and then, using it as a raw powder containing Ni.

In the case of the glass lens mold, high oxidation resistance, high hardness, and high specularity are required; however, the level required for strength is modest. Therefore, mainly for the purpose of achieving high specularity, the amount of Ni is set to an extremely small amount of 0.12 to 0.3 mass %. Here, when the amount of Ni is less than 0.12 mass %, the amount of Ni becomes too small, which leads to a decreased sinterability and then the pore tends to be generated when performing ordinary sintering, and as a result, high specularity can not be obtained. When the amount of Ni exceeds 0.3 mass %, the size of the Ni pool serving as a concave portion at the time of polishing becomes too large to be ignored even if the Ni pool is finely dispersed, and then, high specularity can not be obtained.

It is noted that V is contained in an amount of 30 to 100 mass % with respect to the amount of Ni, and Cr is contained in an amount of 130 to 300 mass % with respect to the amount of Ni, and then, the third phase containing V and Cr is dispersed in the structure of the cemented carbide with a maximum size of 0.3 μm or less. When the contents of V and Cr are less than their lower limits, WC grain growth can not be suppressed, and as a result, the third phase containing V and Cr fails to be finely dispersed and, at the same time, oxidation resistance becomes insufficient. When the contents of V and Cr are larger than their upper limits, the contents becomes significantly large, and then, the third phase fails to be finely dispersed.

In the case of the ultrahigh pressure generation container for neutron diffraction experiment, and the general non-ferromagnetic, corrosion resistant, and wear resistant tool, high strength and high hardness are required; however, high specularity is not required. Therefore, mainly for the purpose of achieving high strength, the amount of Ni is set to 3 to 9 mass %. When the amount of Ni is less than 3 mass %, the amount of the binder phase becomes too small, and then, the strength becomes insufficient. When the amount of Ni exceeds 9 mass %, the hardness becomes insufficient.

It is noted that V is contained in an amount of 5 to 15 mass % with respect to the amount of Ni, and Cr is contained in an amount of 11 to 50 mass % with respect to the amount of Ni, and then, the third phase containing V and Cr is dispersed in the structure of the cemented carbide with a maximum size of 0.3 μm or less. When the contents of V and Cr are less than their lower limits, grain growth can not be suppressed and the strength becomes insufficient, and as a result, the third phase containing V and Cr fails to be finely dispersed. When the contents of V and Cr are larger than their upper limits, the third phase containing V and Cr fails to be finely dispersed, which leads to a decrease in the strength.

It is noted that the reason why the relation between the range of the amount of Ni and the range of the contents of V and Cr is different between the aforementioned two types of tools is that there is a need to reduce the contents of V and Cr because an increase in the amount of Ni leads to an increase in the size of the Ni domain, and then, leads to an increase in the size of the third phase containing V and Cr.

Furthermore, needless to say, it is desirable to add V and Cr each in a carbide state.

The WC based ultra-fine cemented carbide for wear resistant tools, comprising tungsten carbide (WC) and 0.12 to 9 mass % of Ni, wherein V is contained in an amount of 5 to 100 mass % with respect to the amount of Ni, and Cr is contained in an amount of 11 to 300 mass % with respect to the amount of Ni, so that a third phase containing V and Cr is dispersed in a structure of the cemented carbide with a maximum size of 0.3 μm or less, and the cemented carbide has a hardness of 2100 to 2800 HV10 and a size of a Ni pool of 0.3 μm or less, and has an average grain size of WC of 0.3 μm or less has the following advantages: (1) higher processability in comparison with the conventional cemented carbide comprising WC and a solid solution composite carbide phase of TiC-WC; (2) higher productivity in comparison with the WC-W₂C_(x) based cemented carbide and the cemented carbide composed of a single phase of WC; (3) higher oxidation resistance in comparison with the WC-VC-Cr₃C₂—Co based ultra-fine cemented carbide; and (4) high strength even though the WC based ultra-fine cemented carbide of the present invention contains a smaller amount of the binder phase than the conventional WC-VC-Cr₃C₂—Ni based ultra-fine cemented carbide. By using the WC based ultra-fine cemented carbide of the present invention, it is possible to easily produce an aspherical glass lens mold having high specularity, and to safely handle the ultrahigh pressure generation container for neutron diffraction experiment because the container is less radioactive even after its use as described above. Furthermore, the WC based ultra-fine cemented carbide of the present invention can be used for the ultrahigh pressure generation container requiring high strength, and provides a longer lifetime to the non-ferromagnetic corrosion resistant and wear resistant tool.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an optical micrograph of WC-0.2 mass % VC-0.3 mass % Cr₃C₂-3 mass % Ni cemented carbide produced by using ordinary Ni and employing a conventional process of 1440° C.—1 hour sintering and HIP at 1440° C. under 100 MPa. The white phase represents a Ni pool.

FIG. 2 is an optical micrograph of a WC-0.11 mass % VC-0.45 mass % Cr₃C₂-0.2 mass % Ni cemented carbide having WC average grain size of 0.3 μm, prepared by the present invention.

FIG. 3 illustrates an effect of the average grain size of WC on the average size of Ni domain. An open circle represents a case of a WC-(XC)-10 mass % Ni cemented carbide (X represents Cr and/or V) upon sintering under ordinary sintering conditions of 1450° C.—1 hour and then cooling at 0.07° C./s. A filled circle represents a case of the same cemented carbide upon sintering under ordinary sintering conditions of 1450° C.—1 hour and then cooling at 0.23° C./s.

FIG. 4 is a SEM photograph of WC-0.44 mass % VC-1.39 mass % Cr₃C₂-6 mass % Ni cemented carbide having WC average grain size of 0.3 μm, prepared by the present invention. Grains with white or gray contrast represent WC, and a portion with black contrast represents Ni. Although a third phase containing V and Cr can not be reliably confirmed in this magnification due to its fineness, the third phase was confirmed to precipitate in this.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Example 1

A raw powder of WC having an average grain size of 150 nm (calculated on the basis of the BET value) was used and then intensively wet-pulverized for 72 hours by using a ball mill under the conditions that a compound composition was WC-5.5 mass % VC-22.5 mass % Cr₃C₂-10 mass % Ni and weight ratio of a cemented carbide ball serving as a pulverizing medium to the powder was 5 to 1. After the intense pulverization, obtained slurry was dried, and then, was stored as powder A. Then, this powder A was used at 2 mass % and the raw powder of WC of 150 nm was added thereto at 98 mass % so that the entire composition became WC-0.11 mass % VC-0.45 mass % Cr₃C₂-0.2 mass % Ni, and then, wet pulverization was performed for 72 hours under the condition that weight ratio of a cemented carbide ball serving as a pulverizing medium to the powder was 3 to 1. After the wet pulverization, drying was performed. The obtained powder after drying was subjected to cold pressing, vacuum sintering at 1500° C. for 1 hour, and HIP at 1500° C. under 150 MPa for 1 hour in Ar atmosphere, thus the targeted cemented carbide was obtained.

The WC-0.11 mass % VC-0.45 mass % Cr₃C₂-0.2 mass % Ni ultra-fine cemented carbide thus obtained became a material with few Ni pools as shown in FIG. 2. In the obtained cemented carbide, a third phase containing V and Cr was dispersed in a microstructure of the cemented carbide with a maximum size of 0.3 μm or less, the maximum size of Ni was 0.3 μm, and the average grain size of WC was 0.3 μm. The obtained cemented carbide had a hardness of 2200 HV10, transverse-rupture strength of 2.4 GPa, and, when used as an aspherical glass lens mold, had a surface roughness of less than 1 nm Ra, which was smaller than those of the conventional cemented carbides, thereby achieved excellent specularity. Furthermore, weight increase by oxidation after holding in the atmosphere at 800° C. for 1 hour was 170 g/m², which was advantageously smaller than weight increase by oxidation under the same conditions of a commercially available cemented carbide comprising WC and a solid solution composite carbide phase of TiC-WC, 240 g/m².

Here, the measurement method of the average grain size of WC is as follows. That is, SEM observation was performed on the obtained cemented carbide at a fractured surface and a polished-and-etched surface, and then, a three-dimensional average grain size of WC was obtained based on a two-dimensional photograph of microstructure of the polished-and-etched surface by using Fullman's equation.

These results are shown as No. 9 in Tables 1-1 and 1-2, and the results of cemented carbides of No. 8 and No. 10 to No. 13 were similar.

It is noted that the same result was obtained even when the intense wet pulverization with a ball mill was replaced by another intense pulverization with a planetary mill or the like.

Example 2

A raw powder of WC having an average grain size of 150 nm (calculated on the basis of the BET value) was used and then intensively wet-pulverized for 72 hours by using a ball mill under the conditions that a compound composition was WC-0.88 mass % VC-2.78 mass % Cr₃C₂-12 mass % Ni and weight ratio of a cemented carbide ball serving as a pulverizing medium to the powder was 5 to 1. After the intense pulverization, obtained slurry was dried, and then, was stored as powder B. Then, this powder B was used at 50 mass % and the raw powder of WC of 150 nm was added thereto at 50 mass % so that the entire composition became WC-0.44 mass % VC-1.39 mass % Cr₃C₂-6 mass % Ni, and then, wet pulverization was performed for 72 hours under the condition that weight ratio of a cemented carbide ball serving as a pulverizing medium to the powder was 3 to 1. After the wet pulverization, drying was performed. The obtained powder after drying was subjected to cold pressing, vacuum sintering at 1440° C. for 1 hour, and HIP at 1440° C. under 100 MPa for 1 hour in Ar atmosphere, thus the targeted cemented carbide was obtained.

The WC-0.44 mass % VC-1.39 mass % Cr₃C₂-6 mass % Ni ultra-fine cemented carbide thus obtained became a material without Ni pool having a larger size than the average grain size of WC, 0.3 μm, as shown in FIG. 4. The obtained cemented carbide had a hardness of 2160 HV10, a transverse-rupture strength of 4.1 GPa, which offered sufficient strength as an ultrahigh pressure generation container for neutron diffraction experiment. Moreover, the obtained cemented carbide offered far less radiation danger than the conventional WC-Co based cemented carbide and then enabled easy handling because the binder phase was composed of Ni. These results are shown as No. 5 in Tables 1-1 and 1-2, and the results of cemented carbides of No. 1 to No. 4 and No. 6 to No. 7 were similar.

It is noted that the same result was obtained even when the intense wet pulverization with a ball mill was replaced by another intense pulverization with a planetary mill or the like.

Example 3

A raw powder of WC having an average grain size of 70 nm (calculated on the basis of the BET value) was used and then intensively wet-pulverized for 72 hours by using a ball mill under the conditions that a compound composition was WC-1.56 mass % VC-2.70 mass % Cr₃C₂-18 mass % Ni and weight ratio of a cemented carbide ball serving as a pulverizing medium to the powder was 5 to 1. After the intense pulverization, obtained slurry was dried, and then, was stored as powder C. Then, this powder C was used at 50 mass % and the raw powder of WC of 70 nm was added thereto at 50 mass % so that the entire composition became WC-0.76 mass % VC-1.35 mass % Cr₃C₂-9 mass % Ni, and then, wet pulverization was performed for 72 hours under the condition that weight ratio of a cemented carbide ball serving as a pulverizing medium to the powder was 3 to 1. After the wet pulverization, drying was performed. The obtained powder after drying was subjected to cold pressing, vacuum sintering at 1440° C. for 1 hour, and HIP at 1440° C. under 100 MPa for 1 hour in Ar atmosphere, thus the targeted cemented carbide was obtained.

The WC-0.76 mass % VC-1.35 mass % Cr₃C₂-9 mass % Ni ultra-fine cemented carbide having an average grain size of WC of 0.1 μm thus obtained became a material with few Ni pools. The obtained cemented carbide had a high hardness of 2350 HV10, and at the same time, had an excellent transverse-rupture strength of 4.6 GPa, and, when used as an ultrahigh pressure generation container requiring high strength, non-ferromagnetic, corrosion resistant and wear resistant tool, that is, mold, die, punch, cutter, dies, plug, plate, and the like, achieved a lifetime which was twice or more of that of the conventional WC-0.62 mass % VC-1.15 mass % Cr₃C₂-10 mass % Ni submicro-grained cemented carbide (No. 14 in Table 1-1). This invented cemented carbide is shown as No. 1 in Table 1-1 and 1-2 and the results of cemented carbides of No. 2 to No 7 were similar.

It is noted that the same result was obtained even when the intense wet pulverization with a ball mill was replaced by another intense pulverization with a planetary mill or the like.

The characteristics of cemented carbides obtained by the present invention and those of conventional cemented carbides are shown in Table 1-1 and the characteristics of cemented carbides disclosed in Patent Literatures 1 to 6 and Non Patent Literature 3 are shown in Table 1-2. The maximum size of the Ni phase in the cemented carbides of the present invention and in the aforementioned conventional cemented carbides is shown in Table 2.

TABLE 1-1 Average Maximum Content of grain Transverse- Ni Co grain size size of third growth inhibitor rupture Cemented Amount Amount of WC phase (mass %) Hardness strength carbide No. (mass %) (mass %) (μm) (μm) V Cr (HV10) (GPa) Cemented carbide of the 1 9 — 0.1 0.1 0.62 1.17 1.79 2350 4.6 present invention 2 9 — 0.2 0.2 0.53 1.08 1.61 2200 4.4 3 9 — 0.3 0.3 0.45 0.99 1.44 2100 4.2 4 6 — 0.1 0.1 0.42 1.50 1.92 2360 4.3 5 6 — 0.3 0.3 0.36 1.21 1.57 2160 4.1 6 3 — 0.1 0.1 0.45 1.50 1.95 2470 3.8 7 3 — 0.3 0.3 0.38 1.27 1.65 2270 3.6 8 0.3 — 0.1 0.1 0.10 0.45 0.55 2400 2.6 9 0.3 — 0.3 0.3 0.09 0.39 0.48 2200 2.4 10 0.2 — 0.1 0.1 0.11 0.40 0.51 2600 2.4 11 0.2 — 0.3 0.3 0.09 0.34 0.43 2400 2.0 12 0.12 — 0.1 0.1 0.12 0.36 0.48 2800 2.0 13 0.12 — 0.3 0.3 0.10 0.30 0.40 2600 1.8 Conventional 14 10 — 0.5 — 0.50 1.00 1.50 1810 3.7 cemented carbide 15 5 — 0.4 — 0.25 0.50 0.75 2050 2.5

TABLE 1-2 Patent Literature 1-4 — — 1.5 — — *1 *1 (1900) (1.3) Patent Literature 2-1 — 0.06 * 2 0.4 — — 1.0 1.0 2603 — Patent Literature 2-13 — 0.01 * 2 0.35 — — 1.0 1.0 2845 — Patent Literature 2-18 — 0.16 * 2 0.70 — — 1.0 1.0 2818 — Patent Literature 3-1 — 0.4 0.7 — — 0.88 0.88 2620 1.51 Patent Literature 3-6 — 0.4 0.3 — — 0.88 0.88 2650 1.63 Patent Literature 4-A — 0.4 — — *3 *3 *3 2500 — Patent Literature 5-3 5 — 0.6 — *4 0.2 0.7 (1860) (2.16) Patent Literature 6-7 10 — 0.5 — 0.6 — 0.6 (1550) 3.82 Non Patent Literature 3 10 — 0.5 — 0.7 — 0.7 1500 4.2 Non Patent Literature 3 10 — 0.38 — 0.7 — 0.7 1630 4.35 *1 0.5 mass % TiC, 0.5 mass % TaC, the total amount: 1.0 mass % *2 Iron group elements *3 5.5 mass % Mo, 0.4 mass % VC, the total amount: 5.9 mass % *4 0.5 mass % Ta The average grain size of WC is the grain size in the cemented carbide, i.e. sintered body. Hardness value described in parentheses is a value calculated on the basis of HRA and a transverse-rupture strength value described in parentheses is a value calculated on the basis of a value expressed by the unit of kgf/mm².

TABLE 2 Maximum size Cemented of Ni phase carbide No. (μm) Cemented 1 0.1 carbide of 2 0.2 the present 3 0.3 invention 4 0.1 5 0.3 6 0.1 7 0.3 8 0.1 9 0.3 10 0.1 11 0.3 12 0.1 13 0.3 Conventional 14 0.5 cemented 15 1.5 carbide 

1. A tungsten carbide (WC) based ultra-fine cemented carbide suitable for a wear resistant tool, the cemented carbide being based on WC and Ni, and also containing V and Cr, the Ni being contained in an amount of 0.12 to 9 mass %, the V being contained in an amount of 5 to 100 mass % with respect to the amount of the Ni, and the Cr being contained in an amount of 11 to 300 mass % with respect to the amount of the Ni, so that, in addition to a WC phase and a Ni binder phase, a third phase containing the V and the Cr is dispersed in a microstructure of the cemented carbide with a maximum size of 0.3 μm or less, and the cemented carbide has a hardness of 2100 to 2800 HV10 and a maximum size of a Ni pool, or aggregated portion of Ni of the binder phase, of 0.3 μm or less, and has an average grain size of the WC of 0.3 μm or less, WC powder used to make the cemented carbide being obtained by intensively pulverizing the WC and Ni powder used to make the cemented carbide being obtained by intensively pulverizing Ni so that the Ni powder is work-hardened.
 2. An aspherical glass lens mold produced from a tungsten carbide (WC) based ultra-fine cemented carbide suitable for a wear resistant tool, the cemented carbide being based on WC and Ni, and also containing V and Cr, the Ni being contained in an amount of 0.12 mass % to 0.3 mass %, the V being contained in an amount of 30 to 100 mass % with respect to the amount of the Ni, and the Cr being contained in an amount of 130 to 300 mass % with respect to the amount of the Ni, so that, in addition to a WC phase and a binder phase, a third phase containing the V and the Cr is dispersed in a microstructure of the cemented carbide with a maximum size of 0.3 μm or less, and the cemented carbide has a hardness of 2450 to 2800 HV10 and a maximum size of a Ni pool, or aggregated portion of Ni of the binder phase, of 0.3 μm or less, and has an average grain size of the WC of 0.3 μm or less, WC powder used to make the cemented carbide being obtained by intensively pulverizing the WC and Ni powder used to make the cemented carbide being obtained by intensively pulverizing Ni so that the Ni powder is work-hardened.
 3. An ultrahigh pressure generation container for neutron diffraction experiment produced from a tungsten carbide (WC) based ultra-fine cemented carbide suitable for a wear resistant tool, the cemented carbide being based on WC and Ni, and also containing V and Cr, the Ni being contained in an amount of 3 to 9 mass %, the V being contained in an amount of 5 to 15 mass % with respect to the amount of the Ni, and the Cr being contained in an amount of 11 to 50 mass % with respect to the amount of the Ni, so that, in addition to a WC phase and a Ni binder phase, a third phase containing the V and the Cr is dispersed in a microstructure of the cemented carbide with a maximum size of 0.3 μm or less, and the cemented carbide has a hardness of 2100 HV10 to 2700 HV10 and a maximum size of a Ni pool, or aggregated portion of Ni of the binder phase, of 0.3 μm or less, and has an average grain size of the WC of 0.3 μm or less, WC powder used to make the cemented carbide being obtained by intensively pulverizing the WC and Ni powder used to make the cemented carbide being obtained by intensively pulverizing Ni so that the Ni powder is work-hardened.
 4. A non-ferromagnetic corrosion resistant and wear resistant tool produced from a tungsten carbide (WC) based ultra-fine cemented carbide suitable for a wear resistant tool, the cemented carbide being based on WC and Ni, and also containing V and Cr, the Ni being contained in an amount of 3 to 9 mass %, the V being contained in an amount of 5 to 15 mass % with respect to the amount of the Ni, and the Cr being contained in an amount of 11 to 50 mass % with respect to the amount of the Ni, so that, in addition to a WC phase and a binder phase, a third phase containing the V and the Cr is dispersed in a microstructure of the cemented carbide with a maximum size of 0.3 μm or less, and the cemented carbide has a hardness of 2100 to 2700 HV10 and a maximum size of a Ni pool, or aggregated portion of Ni of the binder phase, of 0.3 μm or less, and has an average grain size of the WC of 0.3 μm or less, WC powder used to make the cemented carbide being obtained by intensively pulverizing the WC and Ni powder used to make the cemented carbide being obtained by intensively pulverizing Ni so that the Ni powder is work-hardened. 